Скачать или смотреть How to Make a Superclass of a Template Class Abstract in C++

Discover how to effectively make your superclass of template classes abstract in C++. We'll cover common pitfalls and provide practical solutions for compiling errors!
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Introduction: Making a Superclass Abstract in C++

When working with object-oriented programming in C++, creating an abstract superclass is a common requirement. An abstract class serves as a blueprint for other classes, and it cannot be instantiated on its own. This ensures that any subclass must provide implementations for its abstract methods. However, when dealing with template classes, the process can become tricky, leading to compilation errors if not handled correctly.

In this guide, we'll address a specific scenario where a developer is trying to make a superclass of a template class abstract, but encounters a compilation error. We'll walk through a detailed solution to overcome this hurdle while providing context and explanations to clarify the process.

The Problem

You have a superclass named FieldAccessor, which is a regular class that contains a pure virtual function, marking it as abstract. The intent is for this class to be extended by template classes, specifically PrimitiveFieldAccessor. However, an error arises during compilation, indicating an issue with the syntax for calling pointer-to-member functions.

Compilation Error Encountered:

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This error hints at a fundamental misunderstanding in the way member functions are called when using pointers to member functions, especially in the context of templates.

Understanding the Compilation Error

The compilation error is produced because the syntax used to call the pointer-to-member function isn’t correct. In C++, when you want to call a member function through a pointer-to-member-function, you need to use the following syntax:

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Or, if the object is a pointer, the syntax changes slightly:

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Problematic Code:

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Correct Usage:

You need to utilize the correct syntax to invoke the member function. The corrected forms should be:

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Correcting the Implementation

Now that we understand the cause of the compilation error, let’s take a look at the full adjustments necessary in your implementation file (DriverFunctionSupplierNative.cpp), focusing on the function that retrieves the value.

Modified getValue Function

You'll need to refactor the getValue method in PrimitiveFieldAccessor as follows:

[[See Video to Reveal this Text or Code Snippet]]

Why This Works

The reason for the change lies in how C++ handles virtual functions and templates. The virtual function in your abstract class might prompt the compiler to instantiate the class prematurely to build the virtual table (vtable). Other non-virtual methods, however, get instantiated only upon usage, hence the discrepancy in compilation during template instantiation.

Conclusion

In summary, making a superclass of a template class abstract in C++ is straightforward if you are careful with your syntax, especially concerning pointers to member functions. By understanding the error message and adjusting your implementation accordingly, you can effectively compile your code without issues. Remember to always refer back to the C++ documentation when in doubt about function call syntax, especially when dealing with more complex scenarios like templates and abstract classes.

For any further questions or clarifications, do not hesitate to leave a comment below or explore more about C++ templates and abstract classes in our upcoming posts!